Structural,Electronic And Magnetic Study Of Zirconium-based 2D Materials

With the great success of graphene,two dimensional(2D)layered materials become the research focus of intense due to their unique physical and electronic properties.These promising materials have quite interesting optical and electronic properties are typically diverse due to confinement of electrons,but also due to the weak interlayer interactions.Thus,2D materials are composed of insulators,semiconductors and semimetal.Among various 2Ds,zirconium(Zr)based layered materials are ideal objects because of their superior electrical properties,but still rarely been studied due to the growth challenges.It is very desirable to selectively synthesize high-quality Zr-based 2D materials for further fundmental research as well as future applications.In this thesis,we demonstrate the controllable synthesis and composition engineering of Zr-based dichalcogenides 2D single crystals(ZrS2,ZrSe2 and ZrTe2)via chemical vapor transport(CVT)method,along with the detail structural and chemical characterizations by using different techniques,such as single crystal X-Ray Diffraction(XRD),X-ray photoelectron spectroscopy(XPS),X-ray absorption fine structure spectroscopy(XAFS),scanning transmission electron microscopy(STEM)and scanning tunnelling microscopy(STM).Subsequently,we systematically investigate the electronic properties of Zr-based 2Ds using low temperature transport measurement,angle resolved photoemission spectroscopy(ARPES)combined with first principle density functional theory(DFT)calculations.Meanwhile,their magnetic properties are also studied through physical property measurement system(PPMS)and electron paramagnetic resonance spectroscopy(EPR).The main research activities and results are summarized as follows:We investigated the room temperature ferromagnetism in diamagnetic ZrS2 single crystals with a little amount of Fe dopants.Combined with first principle density functional theory(DFT)calculations,our experimental results confirmed the long range ferromagnetic ordering in semiconductor Fe-doped ZrS2,originating from the Fe spin polarized electron near the Fermi level.The obtained high-quality dilute magnetic semiconductor can be utilized for future spintronic applicationsWe systematically tuned the electronic properties of zirconium diselenide(ZrSe2)via copper(Cu)atomic intercalation.The electrical field effect transistor measurements clearly showed a transition from semiconductor to metallic phase in ZrSe2 single crystals after Cu atoms incorproation,which are further confirmed by ARPES measurements and DFT calculations.The results highlight that atomic intercalation approach may have high potential for realizing transparent electron-doping systems for many specific 2D-based nanoelectronic applications.We controllably used Hafnium(Hf)atoms to substitute on Zr site in ZrSe2 to modulate its intrinsic electronic structures.In contrast to pristine ZrSe2,the ARPES results revealed that the emergence of conduction band at high symmetry M point around the Brillouin zone boundary in Zr1-xHfxSe2,show the metallic state.It can be ascribed due to the effective charge transfer from substituted Hf to Zr atoms that can fill the bottom of conduction band,as a result it can cross the Fermi level.Similarly,the electrical transport measurements further confirm the metallic behaviour of Zr1-xHfxSe2.This study suggests the possibility of the band gap engineering through heavily doped metal in 2D materials,thereby modulating the electronic properties of layered materials for next-generation electronic applications.We selectively synthesized the hexagonal zirconium ditelluride(ZrTe2)and ZrTe2(1-x x)Se2x(0 ? x ? 1),their physical and electronic properties were studied through ARPES combined with DFT calculations.As compare with metallic ZrTe2,the valance band of ZrTe2(1-x)Se2x is degenerated at ? point due to spin-orbit interaction,resulting the steadily band gap opening as the Se concentration increasing.The simulation and ARPES results indicated that the substituted Se atoms on Te site could affect the electronic structure of pristine ZrTe2,leading distinct from metal into semiconductor.This polymorphism in 2D layered materials may enable these materials for future spintronic and valleytronic applications.We further carried out to intercalate chromium(Cr)atoms in ZrTe2,in order to realize their specific physical properties.Interestingly,a superconducting behavior with a T,around 5.2 K was observed in Cr-intercalated ZrTe2,along with an unusual charge density wave behavior.Similarly,we have found the superconductivity transition in Hf-doped ZrTe2 about 4 K.These results suggesting that the efficient lattice distortion or disorder via atomic intercalation or substitution can tuned the physical properties of 2D materials.The coexisting of different electronic phases with tunable physical properties in 2Ds are useful for different electronic applications.